This invention is directed to fiber optic couplers and in particular to low loss access couplers for multimode optical fibers and a method of producing these couplers.
The two main network topology systems for the distribution of optical communication signals using single-strand multimode fibers are the tree distribution system and the star system. For networks with many terminals the tree distribution scheme provides advantages of flexibility in the number and location of the distribution paths or drops and minimizes the amount of fiber used in comparison to a star system. However, a tree network can suffer from an ineffective utilization of the total optical power launched in the trunk feeder if there are many lossy access junctions along the trunk which are encountered in series. The tree scheme can be effectively utilized only if the excess loss above furcation loss at each access junction is made sufficiently small.
Recently two methods for producing low loss access couplers for multimode fibers have been demonstrated. In the first which is described in a publication by Takeshi Ozeki and Brian S. Kawasaki entitled "Optical directional coupler using tapered sections in multimode fibers", Applied Physics Letters, Vol. 28, No. 9, May 1, 1976--pages 528 and 529, twin biconical tapered sections of multimode optical fibers are joined by an optical epoxy to produce a directional coupler. In the second which is described in a publication by M. K. Barnoski and H. R. Friedrich entitled "Fabrication of an access coupler with single strand multimode fiber waveguides",-Applied Optics, Vol. 15, No. 11, November 1976, pages 2629-2630, two sections of multimode fiber are fused side-by-side to form a low loss junction. In both of these structures, the excess loss is in the order of 1 dB.